The present invention is directed generally to reinforced composite articles, and more particularly to methods and apparatus for producing reinforced composite articles.
Concrete and other masonry or cementitious materials have high compressive strength but relatively low tensile strength. When concrete is employed as a structural member, such as in a building, bridge, pipe, pier, culvert, or the like, it is conventional to incorporate reinforcing members to enhance the tensile strength of the structure. Historically, the reinforcing members are steel or other metal reinforcing rods or bars, i.e., xe2x80x9crebarxe2x80x9d. Such reinforcing members may be placed under tension to form prestressed concrete structures.
Although steel and other metals can enhance the tensile strength of a concrete structure, they are susceptible to oxidation. For example, ferrous metal rusts by the oxidation thereof to the corresponding oxides and hydroxides of iron by atmospheric oxygen in the presence of water. When it is poured, concrete is normally at a pH of 12 to 14 (i.e., at high alkalinity) due to the presence of hydroxides of sodium, potassium, and calcium formed during the hydration of the concrete. As long as a pH in this range is maintained, steel within the concrete is passive, which results in long-term stability and corrosion resistance.
Exposure to a strong acid, or otherwise lowering the pH of concrete, can cause steel contained in concrete to be corroded. For example, chlorine ions permeating into the concrete can cause corrosion. Sources of chlorine ions include road salt, salt air in marine environments, and salt-contaminated aggregate (e.g., sand) used in making the concrete. When the reinforcing steel corrodes, it can expand and create internal stresses in the concrete. These internal stresses can lead to cracking, and ultimately disintegration, of the concrete. Moreover, cracking and crumbling concrete exposes additional steel to atmospheric oxygen, water, and sources of chlorine ions.
Such structural damage has become a major problem in a wide variety of geographical areas. For example, bridges and other concrete building infrastructures in northern United States cities are constantly in need of repair because of the salting of roadways after winter snowstorms. Also, bridges leading to the Keys in Florida are continuously exposed to sea air; these bridges are regularly rebuilt because of the short lifespan of the concrete. As another example, buildings in Saudi Arabia and the Middle East, where concrete is typically made using the acidic sand of the region, are often in need of repair.
Various solutions to the corrosion problem of steel rebar have been offered; however, these solutions have been largely unsuccessful. Noncorrosive coatings on the concrete, the steel rebar, or both have been proposed. For example, U.S. Pat. No. 5,271,193 to Olsen et al. proposes a steel-reinforced concrete product, such as a manhole cover, having a coating of a corrosion-resistant gel coat layer and an intermediate layer of fiberglass between the concrete and the gel coat layer. The gel coat layer is described as being a xe2x80x9chardenable polymeric fluid material.xe2x80x9d U.S. Pat. No. 4,725,491 to Goldfein proposes steel rebar members having chemical conversion iron oxide coatings, such as black iron oxide. U.S. Pat. No. 5,100,738 to Graf proposes steel rebar having an outer layer of a synthetic material (e.g., epoxy resin) and an intermediate layer of aluminum or aluminum alloy between the outer layer and the steel. Unfortunately, in general these exemplary coatings tend to be expensive and have received mixed results and acceptance.
There has also been interest in replacing steel with various fiber-reinforced resins. For example, U.S. Pat. No. 5,077,133 to Kakihara et al. proposes an inner filament bundle layer spirally wound around a fiber-reinforced core, a plurality of intermediate filament bundles oriented axially along the core, and an outer filament bundle spirally wound around the core and the other bundles. U.S. Pat. No. 4,620,401 to L""Espxc3xa9rance et al. proposes a fiber reinforced thermosetting resin core and a plurality of continuous fibers helically wound around the core and impregnated with the thermosetting resin. The fiber-reinforced rods proposed in L""Espxc3xa9rance have manufacturing limitations and are difficult to manufacture continuously and rapidly. Additionally, the winding of filaments onto a core tends to reduce the tensile strength of the core and can cause wicking problems.
Other solutions include a corrosion-resistant fiber-reinforced rebar, disclosed in co-pending U.S. patent application Ser. No. 08/467,157, which comprises a fiber reinforced thermoset core and an outer cladding formed of sheet molding compound (SMC), and a three-layered reinforced resin-based composition, described in co-pending and co-assigned U.S. patent application Ser. No. 08/527,976, the disclosures of each of which are hereby incorporated herein by reference in their entireties. These materials are formed into rebar through modified pultrusion processes. Conventional pultrusion processes involve drawing a bundle of reinforcing material (e.g., glass filaments or fibers) from a source thereof, wetting the fibers and impregnating them (preferably with a thermosettable polymer resin) by passing the reinforcing material through a resin bath in an open tank, pulling the resin-wetted and impregnated bundle through a shaping die to align the fiber bundle and to manipulate it into the proper cross-sectional configuration, and curing the resin in a mold while maintaining tension on the filaments. Because the fibers progress completely through the pultrusion process without being cut or chopped, the resulting products generally have exceptionally high tensile strength in the longitudinal (i.e., in the direction the filaments are pulled) direction. Exemplary pultrusion techniques are described in U.S. Pat. No. 3,793,108 to Goldsworthy; U.S. Pat. No. to Fuway; 4,445,957 to Harvey; and U.S. Pat No. 5,174,844 to Tong. Exemplary pultruded articles include tool handles, mine shaft bolts, pipes, tubing, channel, beams, fishing rods and the like. In the patent applications cited above, a pultruded core is surrounded by a molded outer cladding layer formed of a reinforced resin.
Some rebar components are desirably curved or bent in order to follow the contour of the surrounding concrete structures. Unfortunately, one troublesome area for pultrusion processes is the manufacture of such nonlinear articles. Because a typical pultrusion process involves pulling material through an elongated heated die which at least partially cures, and therefore stiffens, the pultruded article, establishing bends or curves in the articles without sacrificing the advantages provided by pultrusion is problematic. As a result, conventional pultrusion processes for making linear rebar have proven to be particularly unsuitable for the production of nonlinear rebar.
In view of the foregoing, it is an object of the present invention to provide an apparatus and an associated method for producing nonlinear components with a pultrusion process.
It is also an object of the present invention to provide a pultruded nonlinear rebar component.
It is an additional object of the present invention to provide a composite material suitable for pultruding into a nonlinear rebar component.
It is yet another object to provide a method and associated apparatus for forming either linear or nonlinear components.
These and other objects are satisfied by the present invention, which provides a precured rebar product (or other reinforced article) that can be formed into either linear or nonlinear rebar as desired. The precured rebar product includes a core of reinforcing fibers impregnated with a resin, a cladding of reinforcing fibers and other reinforcing material (such as ceramic powder or spheres) impregnated with a resin, and an outer shell, typically formed of a material that is sufficiently ductile to be formed into a desired shape, yet is sufficiently rigid to retain its shape once it is formed (such as aluminum or steel). The core and cladding can be formed by conventional pultrusion-type techniques, but the resins of the core and cladding are not completely cured. They are encased in the shell material, which preferably includes deformations into which the resin of the cladding can flow to form the deformations of the rebar. Once the precured rebar product has been produced, it can then be formed into a desired shape (either linear or nonlinear) and can then be completely cured. After curing, the shells can be removed to provide a composite structural rebar component.
An apparatus suitable for producing the aforementioned precured rebar product includes means for forming a core having unidirectionally oriented impregnated fibers, means for forming a cladding having unidirectionally oriented fibers and additional reinforcing material, and means for encasing the core and cladding with a shell of a desired shape. It is preferred that both the core and cladding be formed by pulling the reinforcing fibers through respective resin baths, then forcing them through shaping fixtures. The shell can be applied by also forcing it through a sleeve through which the core and cladding also travel, then forming a seam in the shell material to prevent the resin from escaping. Preferably, if linear rebar is to be formed, the resin is cured xe2x80x9cin-linexe2x80x9d with the formation of the core and cladding. If, instead, nonlinear rebar is to be produced, it is preferred that the precured rebar product be cut to length and bent to the desired shape prior to curing.